Fix two flaky Android WebContentsTests when run with --site-per-process

Both testVisibilityControlsBinding and testChildProcessImportance
tests use getSandboxedChildProcessConnection() to find the renderer
process on which they test their assertions.  They assume there's just
one renderer process and use the first one found, but when these tests
run with --site-per-process, there will also be a spare renderer
process created, which might be randomly picked and cause the tests to
fail flakily on Site Isolation Android FYI bot.  Fix by ensuring the
spare process is disabled for these tests.

Bug: 1137881
Change-Id: I93cc34bf472a72f91a6ec0e9a0ee76029a9b38a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3245483
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#935226}